The measles vaccine is routinely administered to children in two doses according to the immunization schedule. The first dose is given at 12 months of age (1 year), and the second dose is given at 4 years of age. Administering the second dose before the specified age is generally not recommended. However, for children under 4 years old who must travel to an area with an active measles outbreak, the second dose may be given earlier. Adolescents and adults without immunity should also receive two doses of the measles vaccine. There is no harm in individuals who are unsure of their vaccination history receiving the measles vaccine.